In a long term evolution system (Long Term Evolution, LTE), as an emerging mobile long term evolution technology, many new technologies are adopted, and four uplink channels, namely, a physical uplink shared channel (Physical Uplink Shared Channel, PUSCH), a physical uplink control channel (Physical Uplink Control Channel, PUCCH), a physical random access channel (Physical Random Access Channel, PRACH), and a sounding reference signal (Sounding Reference Signal, SRS) are defined. The PRACH is used for random access of a user equipment (User Equipment, UE), and the sounding reference signal is used for measuring uplink channel information.
A signal sent by a UE on a PRACH channel is called a preamble sequence (Preamble Sequence), and the PRACH occupies resources of 6 consecutive resource blocks (Resource Block, RB) in a frequency domain. The preamble sequence is a Zadoff-Chu (ZC) sequence, with a length of 839 points.
A ZC sequence is a classic sequence that satisfies a constant amplitude zero autocorrelation (constant amplitude zero autocorrelation, CAZAC) sequence feature, with a mathematic definition as follows:
when N is an even number,
                    x        u            ⁡              (        n        )              =                            ⅇ                                    -              j                        ⁢                                                  ⁢                                          u                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                                  n                  2                                            N                                      ⁢                                  ⁢        0            ≤      n      <      N        ;and
when N is an odd number,
            x      u        ⁡          (      n      )        =                    ⅇ                              -            j                    ⁢                                          ⁢                                    u              ⁢                                                          ⁢              π              ⁢                                                          ⁢                              n                ⁡                                  (                                      n                    +                    1                                    )                                                      N                              ⁢                          ⁢      0        ≤    n    <          N      .      
In the foregoing formulas, ZC sequences with different values of u are called different physical root sequences. A value range of u is (1, 838).
The ZC sequence has good autocorrelation and cross correlation, and these features of ZC are an important basis for coherent demodulation detection of the PRACH.
The ZC sequence has a good autocorrelation feature: Except that an initial point is N, other points are all Os:
            R      u        ⁡          (      l      )        =                    ∑                  k          =          0                          N          -          1                    ⁢                                    x            u                    ⁡                      (            k            )                          ·                              x            u            *                    ⁡                      (                          k              +              l                        )                                =          {                                                  N              ,                                                          l              =              0                                                                          0              ,                                                          l              =              others                                          
Result amplitudes of cross correlation between ZC sequences are almost all equal to Sqrt(N):
            R      u        ⁡          (      l      )        =                    ∑                  k          =          0                          N          -          1                    ⁢                                    x            u                    ⁡                      (            k            )                          ·                              x            v            *                    ⁡                      (                          k              +              l                        )                                ≈          N      
A sounding reference signal also comes from a change of a ZC sequence. A sounding reference signal sequence traverses a whole sounding reference signal cell frequency band in a certain time period, and the sounding reference signal cell frequency band is configured by a higher layer, and is related to a bandwidth. In this way, a conflict inevitably occurs between a frequency domain position of the PRACH and a frequency domain position of the sounding reference signal sequence.
When coherent demodulation is performed on a PRACH sequence, a difference between a length of the sounding reference signal sequence and a length of the PRACH sequence results in that the sounding reference signal sequence may seriously damage the cross correlation of the ZC sequence during the coherent demodulation. As a result, whether a peak value after the PRACH sequence is correlated is an autocorrelation value of the PRACH sequence or a cross correlation value introduced due to the sounding reference signal sequence cannot be identified.
In an actual LTE network, a time-frequency domain position of a sounding reference signal of a neighboring cell or a local cell may overlap with a time-frequency domain position of a PRACH of the local cell. The sounding reference signal sequence and a PRACH preamble sequence both belong to ZC sequences, and the two are different in length, so that as described above, reflected cross correlation is not good. When there is no access from a user, the sounding reference signal of the neighboring cell or the sounding reference signal of the local cell incurs interference to PRACH detection of the local cell, and a serious false alarm may be generated in the PRACH detection.
For this case, the present invention provides a method and an apparatus for demodulating a physical random access channel signal, so as to restrain the interference from the sounding reference signal and reduce false alarms in the PRACH detection.